The heating system of the invention applies notably to nuclear power stations and, in particular, to power stations provided with a boiling water reactor (BWR), but can also be applied to other types of thermal electric power station. The invention more particularly relates to the circuits for recovering heat between, on the one hand, the outlet of at least one condenser and, on the other hand, the inlet of a steam generator system of a power station.
In present-day thermal electric power stations optimizing the water heating circuit is of crucial importance, particularly as far as reducing energy costs are concerned.
The key problem is that it is necessary to convey a flow of water to the inlet of a steam generator system at a given temperature while at the same time making maximum reuse of the energy of the water in steam or condensed form at all stages of the treatments. The issue is therefore one of minimizing the losses of heat energy and of optimizing reuse in the overall operation of a power station.
It is necessary to consider various aspects when optimizing the energy efficiency of a thermal electric power station. In particular, a power station has a number of constraints on the structural integration of the various elements of which it is made up and this means that certain compromises have to be made.
In this regard, the choice of configuration imposes certain safety constraints between the various elements of a thermal electric power station. The safety/efficiency compromise sometimes leads to a loss of heat energy and/or of efficiency in the energy circuit.
FIG. 1 depicts a conventional design of a thermal electric power station comprising a steam generator system 1, a set of high-pressure turbines 8, a set of medium-pressure turbines 9, and a set of low-pressure turbines 10. There is conventionally also an alternator 11 and a condenser 6. A system provides a flow of cooling water to the condenser 6.
The steam generator system 1 and the high-pressure, medium-pressure and low-pressure turbines, the alternator 11, the external circulation circuit 300 and the condenser 6 make up the key elements of the primary circuit of a power station. In some instances, the medium-pressure and low-pressure turbines may be combined.
On the outlet side of the condenser 6, a circuit for extracting water condensed from water extracted from the condenser 6 by a pump 4 comprises a purification system 35, denoted SP, otherwise known as a “polishing system”, followed by a heating circuit made up of several sets of heaters.
The principle relies on recovering some of the residual heat from the steam tapped off at chosen points in the turbine for the purposes of heating up the water fed to the steam generator system. The steam inlets 20, 21 and 23 allow the water of the circuit 30 to be heated up gradually to ensure a flow of water reinjected into the inlet side of the steam generator system 1 at the desired temperature.
The heaters LP1, LP2, LP3, LP4, the feed tank, denoted BA, and the group of heaters denoted HP are mounted in series with respect to the flow of water extracted from the condenser 6 so as to optimize the thermodynamic water-heating cycle. In a conventional configuration, a cooler 7, denoted RC, is positioned upstream of the heating circuit to cool condensate from the heater LP3 before it is returned to the condenser 6.
In a conventional way, for system architecture reasons, a first set of heaters is generally incorporated into a structure comprising the condenser 6 and the low-pressure turbine 10. In the example illustrated, this first set comprises the heaters LP1 and LP2.
A second set of heaters, comprising the heaters LP3 and LP4, is arranged generally outside of the structure comprising the condenser 6.
In general, prudent rules of design dictate that the condensate 100 coming from this second set cannot be recovered directly in the first set of heaters incorporated into a structure comprising the condenser 6 and the low-pressure turbine 10.
Because the condensate 100 coming from the second set of heaters cannot be conveyed directly to the first set, the conventional solution is to cool this condensate before returning it to the condenser 6, in order to avoid significant losses of heat energy.
In this type of configuration, the condensate 100 coming from the second set is injected into the cooler 7 in order to return colder water to the condenser 6 via the outlet 13 of the cooler 7.
There is, however, a significant loss in energy efficiency in the function of heating up the water in the circuit 30 because of the large temperature difference notably between the outlet of the cooler 200 and the return 100.
There are solutions that make it possible to maximize the energy balance when heaters are mounted in cascade. For example, one known alternative is to fit a condensate recovery pump recovering condensate from one particular heater and reinject it into the feedwater circuit downstream of that same heater. Such a system does indeed allow recovery directly in the water cycle at a similar temperature level, therefore maximizing energy efficiency by minimizing temperature differences. However, this solution has several potential disadvantages.
First of all, it adds additional equipment to the water system, particularly pumps, which have a cost, which require space in which to install them, and which demand a certain level of maintenance. Furthermore, the condensate pumped by a recovery pump does not pass through the purification system SP that purifies the water extracted from the condenser, thus reducing the chemical quality of the water in the circuit.
A second known alternative is systematically to cascade the condensate from one heater into the heater of lower rank. As discussed earlier, this solution cannot prudently be applied to heaters incorporated into a structure comprising the condenser 6 and the low-pressure turbine 10 because these tappings are not fitted with nonreturn valves and the backflow of a mixture of cold revaporized water and condensate to the turbine, notably in the event of a sudden sharp pressure drop, could lead to turbine blade damage.
Therefore, the configuration of fitting a drain cooler 7 upstream of the first set of heaters LP is generally the one adopted for reasons of reliability, ease of maintenance and water quality, to the relative detriment of energy efficiency.